A normal image is typically captured with a camera having a 30-70 degree field of view. Special cameras and lenses can produce much wider fields of view, and by using software multiple images can be stitched together into even wider fields of view all the way to a complete sphere. In addition, such images can be generated using a virtual camera viewing a virtual 3D world.
An image can be thought of as a two dimensional set of rays or directions passing through a small pinhole representing the viewpoint of the image. These rays are mapped to a 2D surface for display. The mapping for standard images is created through a “perspective” transformation involving projecting the rays onto a planar surface. This is called a perspective projection and has the advantage of keeping all straight lines straight. Unfortunately perspective projections begin to create large distortions as the field of view passes 90 degrees and as it approaches 180 degrees the distortion becomes infinitely large. Thus, other projections have been developed, such as projections onto cylindrical or spherical surfaces (i.e., projecting onto a curved surface). These projections are “unwrapped” to the plane of a screen for display. This unwrapping allows very wide angle images to be displayed but at the cost of a certain flattening of the result and making straight lines in the world appear curved.
When viewing wide angle images one often wants to be able to pan and zoom to view the entire scene or to examine a smaller portion of the image to appreciate the details. This is especially true when the image is a high resolution image. New high resolution cameras and software stitching technologies allow the creation of very high resolution wide angle images up to billions of pixels. Viewing such images on a computer screen can leverage the high resolution by allowing a user to zoom in (narrow the field of view) and zoom out (widen the field of view) and pan about the image while still utilizing the full resolution of the screen.
A problem arises in the foregoing zooming in and out of a wide angle image in that if the image data is displayed as a perspective projection, zooming out to see more of the scene will result in an image that is distorted at its edges when the field of view exceeds about 70 degrees. On the other hand, if the image data is displayed as a cylindrical or spherical projection, zooming in will produce unacceptable results in comparison to a perspective projection, owing to the aforementioned flattening and curving of straight lines in the displayed image.